1. Field of the Invention
The present invention relates to synthetic fibers used in the manufacture of non-woven fabrics. In particular, the present invention relates to polypropylene fibers, intended for such use, processes of producing polypropylene fibers, compositions for producing polypropylene fibers, non-woven fabrics produced with polypropylene fibers, and articles containing polypropylene fibers. More specifically, the fibers of the present invention are producible using polymer admixtures so as to enable control of desired properties of the fibers.
2. Background Information
The production of polypropylene fibers and filaments usually involves the use of a mix of a single polymer with nominal amounts of stabilizers and/or antioxidants and pigments. The mix is melt extruded into fibers and fibrous products using conventional commercial processes. Non-woven fabrics are typically made by making a web of the fibers, and then thermally bonding the fibers together where they meet. More specifically, staple fibers are converted into non-woven fabrics using, for example, a carding machine, and the carded fabric is thermally bonded. The thermal bonding can be achieved using various heating techniques, including heating with heated rollers and heating through the use of ultrasonic welding.
Conventional thermally bonded non-woven fabrics exhibit good loft and softness properties, but less than optimal cross-directional strength, and less than optimal cross-directional strength in combination with high elongation. The strength of the thermally bonded non-woven fabrics depends upon the orientation of the fibers and the inherent strength of the bond points.
Over the years, improvements have been made in fibers which provide stronger bond strengths. However, further improvements are needed to provide even higher fabric strengths to permit use of these fabrics in today's high speed converting processes for hygienic products, such as diapers and other types of incontinence products. In particular, there is a need for a thermally bondable fiber and a resulting non-woven fabric that possess high cross-directional strength and high elongation.
Further, there is a need to produce thermally bondable fibers that can be produced under varying conditions of draw, by utilizing different draw ratios and temperatures, while achieving superior cross-directional strength, elongation and toughness properties in combination with fabric uniformity and loftiness. In particular, there is a need to obtain fibers that can produce carded, calendared fabrics with cross-directional properties on the order of 650-700 g/in, with an elongation of 140-180%, and a toughness of 480-700 g/in for a 20 g/yd.sup.2 fabric bonded at 250 ft/min.
A number of patent applications, as referred to above, have been filed by the present inventor and by the present assignee which are directed to improvements in polymer degradation, spin and quench steps, and extrusion compositions that enable the production of fibers having an improved ability to thermally bond accompanied by the ability to produce non-woven fabric having increased strength, elongation, toughness and integrity.
In particular, the above-referred to Kozulla Application No. 07/474,897, filed Feb. 5, 1990, now abandoned, application Ser. No. 07/887,416, filed May 20, 1992, now U.S. Pat. No. 5,281,378, application Ser. No. 07/683,635, filed Apr. 11, 1991, now U.S. Pat. No. 5,318,735, application Ser. No. 07/836,438, filed Feb. 18, 1992, now abandoned, and Ser. No. 07/939,857, filed Sep. 2, 1992, now U.S. Pat. No. 5,431,994, are directed to processes for preparing polypropylene containing fibers by extruding polypropylene containing material having a molecular weight distribution of at least about 5.5 to form hot extrudate having a surface, with quenching of the hot extrudate in an oxygen containing atmosphere being controlled so as to effect oxidative chain scission degradation of the surface. For example, the quenching of the hot extrudate in an oxygen containing atmosphere can be controlled so as 4o maintain the temperature of the hot extrudate above about 250.degree. C. for a period of time to obtain oxidative chain scission degradation of the surface.
As disclosed in these applications, by controlling the quenching to obtain oxidative chain scission degradation of the surface, the resulting fiber essentially contains a plurality of zones, defined by different characteristics including differences in melt flow rate, molecular weight, melting point, birefringence, orientation and crystallinity. In particular, as disclosed in these applications, the fiber produced by the delayed quench process includes an inner zone identified by a substantial lack of oxidative polymeric degradation, and an outer zone of a high concentration of oxidative chain scission degraded polymeric material, whereby there is an inside-to-outside increase in the amount of oxidative chain scission polymeric degradation. In other words, the quenching of the hot extrudate in an oxygen containing atmosphere can be controlled so as to obtain fibers having a decreasing weight average molecular weight towards the surface of the fiber, and an increasing melt flow rate towards the surface of the fiber. Moreover, the inner, core zone has a melting point and orientation that is higher than the outer surface zone.
Further, the above referred to Gupta et al. application Ser. Nos. 08/003,696, 07/943,190 and 07/818,772 are directed to processes for spinning polypropylene fibers, and the resulting fibers and products made from such fibers. The processes of the Gupta et al. applications include melt spinning a polypropylene composition having a broad molecular weight distribution through a spinnerette to form molten fibers, and quenching the molten fibers to obtain thermally bondable polypropylene fibers.